The present exemplary embodiments relate to nanostructured catalyst layers. It finds particular application in conjunction with Polymer Electrolyte Fuel Cells (PEFCs), and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Polymer Electrolyte Fuel Cells (PEFCs) are usually fueled with hydrogen or methanol. The hydrogen-air fuel cell (HAFC) is currently considered as the power source of choice for vehicular applications thanks to its superb power density, lifetime and short start-up time. Due to problems with hydrogen storage, another type of PEFC, Direct Methanol Fuel Cell (DMFC), is also investigated for portable and vehicular applications. DMFCs have already entered the market for portable applications, such as military radio stations and laptop computers.
Since PEFCs typically operate at temperatures below 100° C., they require catalysts to achieve practically meaningful power density, particularly in the case of oxygen reduction (ORR) and methanol oxidation (MOR) reactions. Currently, all catalysts used in PEFCs are platinum-based. The current state-of-the-art PEFCs use 0.2-0.4 mg/cm2 of Pt for air cathode, 0.05 mg/cm2 of Pt for hydrogen anode, 0.2 mg/cm2 of Pt+Ru for reformate gas (H2+100 ppm CO) anode, and 4.0 mg/cm2 of Pt+Ru for methanol anode, and the reported performances at 80-100° C. with 1 atm of air are ca. 0.35-1 g/kW for pure hydrogen-fed HAFC, and 20-80 g/kW for DMFC. Published analyses for different types of automotive HAFCs estimate the cost of catalyst as 50-250 $/kW or 20-35% of the fuel cell stack cost. These numbers alone are higher than the 2015 DOE's goals for the complete fuel cell power plant including hydrogen storage, i.e. $30/kW, which is cost competitive the internal combustion engine technology. The situation looks even less promising in the case of automotive DMFCs.
The high cost of PEFCs is not the only problem that impedes their broad use; the scarcity of Pt metal is even a larger concern. Current fuel cell powered automobiles require ca. 60 g of Pt compare to 2-5 g of Pt for internal combustion engine vehicles. According to various estimates, Pt production will be able to meet the demand of projected FCA market growth, only if the required amount of Pt is reduced 4-5 fold, to 15 g/vehicle or to 0.2 g/kW by 2015 and to lower values later. An even more drastic requirement a 10-fold reduction of Pt loading, was proposed as a necessary condition for a large-scale market penetration of automotive fuel cells by a Nissan researcher. M. Arita; Technical Issues of Fuel Cell Systems for Automotive Applications. Fuel Cells 2 (2002) 10-14.
The commercial viability of Polymer Electrolyte Fuel Cells (PEFCs) for large-market applications requires, among other factors, a substantial reduction of platinum metals loading. The state-of-the-art design of the catalytic layer in PEFCs based on carbon-supported ca. 3 nm Pt nanoparticles suffers from three main drawbacks. The first is a poor utilization of Pt nanoparticles, since 25-50% of catalyst particles are buried in carbon agglomerates and, therefore, are not accessible to reactants or protons. The second drawback is a large fraction (50-60%) of subsurface Pt atoms, which do not participate in electrocatalysis. The third is the significant thickness of the catalytic layer (ca. 10 μm), which prevents efficient mass-transport throughout the layer.
Proposed herein is a conceptually new design of the catalytic layer based on recently developed nanofabrication principles.